A suspension unit for a vehicle in which a hydraulic shock absorber is arranged, in parallel with a suspension spring, between a car body and an axle is well known.
Further, Japanese Patent Laid-Open Publication No. 5-44758A has disclosed a suspension unit in which magnet coils are built in a hydraulic shock absorber. In this suspension unit, coils are attached to a cylinder of the hydraulic shock absorber and magnets are attached to a piston rod, respectively, and an electric current is applied to the coils, thereby generating driving force (electromagnetic force) along the direction of a stroke of the piston rod so as to control the quantity of telescopic motion of the suspension unit according to the traveling condition of a vehicle.
However, in such a suspension unit in which magnet coils are built in a hydraulic shock absorber, a hydraulic pressure, a power source, and the like are required, whereby it is complicated in structure and it is disadvantageous in respect of the costs.
On the other hand, a new electromagnetic shock absorber which does not require a hydraulic pressure, an air pressure, a power source, or the like is under study. Such an electromagnetic shock absorber is basically constituted as shown in an exemplified model of FIG. 6.
In this electromagnetic shock absorber, telescopic motion of the shock absorber is converted into rotary motion utilizing a ball screw mechanism and a motor is driven due to the rotary motion, and the telescopic motion of the shock absorber is damped by resistance generated by motor as an electromagnetic force.
A motor 50 is supported by a supporting frame 30, and there is provided a traveling frame 40 which is guided in such a manner that the traveling frame 40 can freely slide with respect to the supporting frame 30. Between a screw shaft 46 and a ball nut 47 which constitute a ball screw mechanism 45, the ball nut 47 is attached to the above-mentioned traveling frame 40, and the screw shaft 46 to be spirally engaged with the ball nut 47 is coaxially connected with a rotary shaft 51 of the above-mentioned motor 50 through a coupling 55.
The supporting frame 30 has an upper bracket 31, a lower bracket 33, and an intermediate bracket 32 which is located between the upper bracket 31 and the lower bracket 33. The supporting frame 30 is constituted in such a manner that these brackets are connected with each other by means of a plurality of connecting rods 34. The above-mentioned screw shaft 46 is rotatably supported, by a bearing 35 installed at the intermediate bracket 32, in such a manner that the screw shaft 46 goes through the bearing 35.
The traveling frame 40 has an upper bracket 41, a lower bracket 42, and a plurality of guide rods 43 which connect these brackets 41 and 42. The guide rods 43 of the traveling frame 40 slidably go through the lower bracket 33 of the above-mentioned supporting frame 30, whereby the guide rods 43 guide the traveling frame 40 in such a manner that the traveling frame 40 can slide in parallel with the screw shaft 46.
The above-mentioned ball nut 47 is attached to the upper bracket 41, and a large number of balls are arranged along a thread groove inside the ball nut 47 although these balls are not shown in the drawing. The screw shaft 46 is spirally engaged with the ball nut 47 through the large number of balls.
When the ball nut 47 together with the traveling frame 40 moves along the screw shaft 46, rotary motion is applied to the screw shaft 46 by the ball screw mechanism 45.
If the electromagnetic shock absorber is interposed between a car body and an axle, for example, and is utilized as a suspension of the car, a mounting bracket 36 of the supporting frame 30 which is located above the motor 50 and at an upper end of the electromagnetic shock absorber will be connected on the side of the car body, and a mounting eye 44 which is provided at the lower bracket 42 of the traveling frame 40 at a lower end of the electromagnetic shock absorber will be connected on the axle side.
When vibration inputs into the electromagnetic shock absorber from the surface of a road and the ball nut 47 makes linear motion in the direction of an arrow X together with the traveling frame 40, the screw shaft 46 makes rotary motion at that position due to spiral engagement of the thread groove of the screw shaft 46 and the balls which are arranged along the thread groove inside the ball nut 47.
The rotary motion of the screw shaft 46 is transmitted, as rotary motion of a rotary shaft 51 in the direction of an arrow Y, through the coupling 55 attached to an upper end of the screw shaft 46, thereby rotating the motor 50.
In the motor 50, for example permanent magnets are arranged at a rotor of the motor 50, and coils of a stat or of the respective magnetic poles short-circuit directly to each other or the coils are connected via a control circuit so that desired electromagnetic force can be obtained. Thus, with the progress of rotations of the rotor of the motor 50, electric currents flow through the coils due to induced electromotive force, and the electromagnetic force which arises resulting from the flow of electric currents becomes torque to oppose against the rotations of the rotary shaft 51 of the motor 50.
Additionally, it is possible to freely change the strength of rotational torque which is based on the electromagnetic force and opposes against the direction of rotations of the rotary shaft 51 by changing the strength of resistance due to the control circuit which is connected with the coils.
Electromagnetic torque which becomes resistance against the rotations of the rotary shaft 51 restrains the rotations of the above-mentioned screw shaft 46. After all, the torque operates as resistance to restrain linear motion of the ball nut 47 of the ball screw mechanism 45, that is, as damping force against the vibration which puts into the electromagnetic shock absorber.
However, with respect to an electromagnetic shock absorber constituted such that the screw shaft 46 is directly connected with the rotary shaft 51 of the motor 50 by the coupling 55 and the rotary motion of the shaft 46 is transmitted to the motor 50, it is feared that the following problems may arise if the electromagnetic shock absorber is actually applied to a vehicle.
First, characteristics of the damping force which is generated by the electromagnetic shock absorber are taken into consideration. With the progress of the linear motion of the ball nut 47, the screw shaft 46 rotates and the rotary motion is transmitted to the motor 50. Because the moment of inertia of the rotor inside the motor 50 is relatively large, its influence on damping force cannot be ignored.
Here, a description as to how the moment of inertia affects the above-mentioned damping force will be given.
The damping force generated by the electromagnetic shock absorber, namely, the resistance (load) against the telescopic motion is approximately the sum total of the moment of inertia of the rotor of the motor, the moment of inertia of the screw shaft, and the electromagnetic resistance generated by the motor. Because angular acceleration of the rotary shaft of the motor is proportional to acceleration of the telescopic motion of the shock absorber, the moment of inertia of the rotor is proportional to the acceleration of the telescopic motion of the shock absorber.
As described above, the moment of inertia of the rotor is proportional to the acceleration of the telescopic motion of the shock absorber and therefore the damping force which is not based on the electromagnetic force of the motor is generated against the force in an axial direction of the shock absorber which is input from the surface of a road into the shock absorber. Especially if sudden force in an axial direction is input, greater damping force, namely, resistance against vibration will be generated by the rotor accordingly. This exceedingly high damping force does not absorb the vibration and the vibration is directly transmitted to the side of a car body as it is.
Therefore, the damping force due to the moment of inertia of the rotor of the motor always arises prior to generation of the damping force which depends on the electromagnetic force of the motor. Moreover, because the moment of inertia of the rotor is relatively great as described above, if it is possible to exclude or restrain the influence which the moment of inertia of the rotor exercises on the damping force, vibrational absorption capacity will be increased accordingly. This makes vehicles more comfortable to drive.
Especially, referring to the electromagnetic shock absorber's controllability over damping force, it is difficult to control the damping force which arises resulting from the moment of inertia of the rotor of the motor closely relevant to the acceleration of telescopic motion of the above-mentioned shock absorber. Thus, it is preferable that the above-mentioned moment of inertia is less influential.
Next, durability of the motor 50 will be taken into consideration. According to input speed of pushing up force from a road surface, vibration, or the like which is applied from the surface of a road to the electromagnetic shock absorber while a vehicle is traveling, the traveling frame 40 travels and the ball nut 47 of the ball screw mechanism 45 makes linear motion at the same speed as the traveling speed of the traveling frame 40. The screw shaft 46 also rotates at a speed in proportion to the speed of the linear motion, and the rotary shaft 51 of the motor 50 also rotates at the same speed as that of the screw shaft 46.
In this case, when the above-mentioned input speed of vibration or pushing up forth is suddenly increased, it is possible to temporarily exceed an allowable rotational speed of the motor 50. Especially if rapid telescopic motion is commenced when the shock absorber is in a stationary state or when slow telescopic motion is in progress, the rotational speed of the motor will extremely increase in a moment. In this case, a calorific value of the coils of the motor 50 will be great, and the calorification will induce a chemical change or the like in insulating coating of conducting wires which form the coils. This will lead to the deterioration of insulation performance. As a result, it is feared that electric leakage may occur and the motor itself may be damaged.
The motor 50 is more expensive than other parts of the electromagnetic shock absorber. Therefore, it is desirable to make every effort to prevent the motor 50 from being damaged.